The present invention relates to an ultrasonic diagnosis device which can diagnose various diseases of blood vessels using an ultrasonic probe which can be inserted into blood vessels.
A technique for imaging the status of blood vessels has become indispensable for diagnosis and operation of various diseases of blood vessels such as arterial sclerosis and aneurysm. In addition to the observation of blood vessel images using the imaging techniques, quantitative measurement of the cross-section of blood vessels and the thickness of blood vessel walls will provide objective standards for evaluating the shape of blood vessels which enables very effective diagnosis and postoperative evaluation of artificial blood vessels. Also, since the blood vessels having a disease are greatly different from healthy blood vessels in dynamical characteristics, the measurement of the dynamical characteristics in addition to the imaging and quantitative measurement will enable more effective diagnosis.
Conventional devices for imaging the shape of blood vessels include X-ray devices, X-ray CT devices and MRI (Magnetic Resonance Imaging) devices for angiography, ultrasonic diagnosis devices for laminagraphy and angioscopes utilizing a fiberscope.
In angiography using a X-ray device, contrast medium are injected into blood vessels in order to facilitate X-ray diagnosis. Though this method is widely used in clinical medicine, it has shortcomings that it is very uncomfortable for patients to whom the contrast medium is injected and that the blood vessel shape information available with this method is two-dimensional observation limited to the periphery of blood vessel caves. With X-ray CT devices or MRI devices, the cross-sectional images of blood vessels can be obtained without the contrast medium. However, the cross-sectional images obtained by these devices are of relatively low resolution and insufficient for the observation of blood vessel shape.
Ultrasonic diagnosis devices enable non-invasive and real-time observation of blood vessels. However, the observation is limited to the blood vessels which can receive the ultrasonic wave coming from the surface of the body. In addition, the resolution of these devices is low. In angioscopes which is inserted into blood vessels for shape observation, transparent liquid is rapidly injected to enable observation and recording of the surface of blood vessel caves. However, this method has shortcomings in that the period during which the injected liquid is sufficiently transparent, i.e. the period for observation is short, and that the observation is limited to the surface of blood vessel caves.
Thus, the devices described above are employed only for the detection of the extreme abnormality of blood vessel shape such as hemadostenosis, for the reasons that the shape information they provide is two-dimensional, that their observation range and resolution are insufficient and that continuous observation is not possible.
Ultrasonic diagnosis technologies which obtain ultrasonic images of the cross-sectional shape of blood vessels using an ultrasonic probe have been developed recently. For example, Japan Pat. No. 62-270,140 (U.S. Pat. No. 4,794,931) discloses a device to obtain ultrasonic images of the cross-sectional shape of blood vessels wherein, an ultrasonic oscillator is provided on the end of an ultrasonic probe (catheter) which is inserted into blood vessels. The ultrasonic oscillator generates ultrasonic waves for diagnosis and receives echoes from the location to be observed. The ultrasonic waves are transmitted or received in an axial direction of the probe using the ultrasonic oscillator or an ultrasonic mirror.
However, though this device provides two-dimensional cross-sectional images of blood vessels, it does not provide longitudinal images in the axial direction or three-dimensional images.
Also, this device does not enable quantitative measurement of the cross-sectional area of blood vessels, the thickness and smoothness of blood vessel walls. It does not serve sufficient diagnosis because its measurement is only qualitative measurement to observe significant changes in blood vessel shape such as hemadostenosis by imaging blood vessels.
It is very much effective in blood vessel diagnosis, especially in the diagnosis of arteriosclerosis, to measure the dynamical characteristics of blood vessels that is the elasticity of blood vessels. The reason for this is that the elasticity of blood vessels is closely related to the shape of blood vessels and therefore the change in the quality of blood vessel organ is reflected in the elasticity of blood vessels prior to the change in blood vessel shape, and that the elasticity of blood vessels is the most quantitative parameter to be used for evaluation of the hardness of blood vessels.
An ultrasonic method has been used to obtain the elasticity of blood vessels. In the ultrasonic method, the elasticity of blood vessels is obtained by calculation based on the diameter or cross-sectional area of blood vessel walls, the change in them in response to pulsation and the thickness of blood vessels which are measured by non-invasively directing ultrasonic waves to blood vessels from the surface of the body.
The principle of measurement is the same for all of the various methods to obtain the elasticity of blood vessels which have been proposed. It is to measure the change in the internal pressure of blood vessels caused by pulsation, that is the rate of blood vessel deformation against the pulse pressure. For example, when blood vessels have been hardened by arteriosclerosis, the change in blood vessel hardness can be identified because there is less deformation of blood vessels compared to the normal deformation against the same pulse pressure. Among the various kinds of blood vessel elasticity, the blood vessel elasticity Ep given by equation (1) below is frequently used to evaluate the degree of the arteriosclerosis of the blood vessels which are not extracted. EQU Ep=.DELTA.P/(.DELTA.D/D) (1)
where .DELTA.P is pulse pressure, .DELTA.D is the diameter of blood vessels and .DELTA.D is the change in diameter in response to pulsation.
The blood vessel elasticity Ep given by the above equation (1) is applicable only when the cross-sectional shape of the blood vessels is considered to be circular. Therefore, true evaluation is not available in case of blood vessels with a changed shape On the contrary, the blood vessel elasticity Ep' obtained from the cross-sectional area of blood vessels is a useful standard which serve the evaluation of blood vessel hardness almost independently of the change in shape. The blood vessel elasticity Ep' can be expressed by equation (2) below where S is the cross-sectional area of blood vessels and .DELTA.S is the change in the cross-sectional area of blood vessels in response to pulsation. EQU EP'=2.DELTA.p/(.DELTA.S/S) (2)
Note that Ep'=Ep/2 in case of a cross-section which can be regarded to be circular.
The blood vessel elasticity Ep or Ep' is the elasticity dependent not only on the hardness of blood vessels but also on the thickness of blood vessel walls and it is not possible to evaluate the hardness of blood vessels themselves. In order to evaluate the hardness of blood vessels, it is required to obtain Young's modulus by measuring the thickness of blood vessel walls. If Young's modulus is available, it can be determined which causes the change in the hardness of blood vessels due to arteriosclerosis, the change in quality or thickness of blood vessels.
As blood vessel elasticity which is equivalent to Young's modulus, incremental elasticity Einc has been suggested. The incremental elasticity Einc can be calculated by equation (3) below where the thickness of blood vessel walls is represented by h. EQU Einc=Ep (1-.delta..sup.2)/(2h/D) (3)
In the above equation .delta. stands for Poisson's ratio which is 0.5 in incompressible blood vessels.
Thus, in order to obtain blood vessel elasticity, it is required to measure the diameter or cross-sectional area of a blood vessel, the change in the diameter or cross-sectional area in response to pulsation, the thickness of blood vessel walls and pulse pressure. These values shall be simultaneously measured at the same location and measurement of high sensitivity is required for the change in response to pulsation which is microscopic.
In the above ultrasonic method, however, the pulse pressure .DELTA.P shall be substituted by the value measured at brachium using a cuff-type hemomanometer. The value will obviously be different from the pulse pressure at the location to be measured and therefore, the calculated blood vessel elasticity will include an error. Also, the measurement is limited to the location which can receive ultrasonic waves. Further, since the diameter of a blood vessel and change in the diameter in response to pulsation is measured using a part of the cross-section of the blood vessel, an error will result when the cross-section cannot be assumed to be circular.
A device to obtain the blood vessel elasticity using an intra-blood-vessel ultrasonic probe is disclosed in Japan Pat. Laid Open No. 63-317130.
This device enables to measure the pulse pressure at the location to be measured to obtain accurate blood vessel elasticity.
With this device, however, it is not possible to diagnose the shape of blood vessels because it does not provide cross-sectional images of blood vessels. Also, since this device cannot perform real-time measurement of the cross-sectional area of blood vessels, there is the same possibility of an error in the measurement of blood vessel elasticity as that in the non-invasive ultrasonic irradiation from the surface of the body in case of blood vessels with a changed shape.